This invention relates to a magnetic memory device for memorizing information by the use of magnetic domains generated in a magnetic medium having an easy axis of magnetization orthogonal to a principal surface thereof.
In each conventional magnetic memory device of the type described, magnetic bubbles, namely, cylindrical domains are used as magnetic domains and transferred along predetermined propagation patterns by a magnetic field. Various attempts have been made to raise up a storage density of the magnetic memory device. For example, a device is proposed which has the propagation patterns formed by the use of an ion-implantation technique. This method is beneficial to memorize magnetic bubbles of a small size at a high density.
However, the storage density is dependent on a bubble size even when the propagation patterns are formed by the ion-implantation technique. It has been said that an attainable minimum bubble size is about 0.3 microns or micrometers so far as a garnet material is concerned. No discovery has ever been made of a practical material which is capable of generating magnetic bubbles less than 0.3 micron meter. Accordingly, it has been deduced that the minimum bubble size of 0.3 micron is an unsurmountable limit and that the upper limit of the storage density is determined by the above-mentioned minimum bubble size.
In AIP Conference Proceedings No. 24, pp. 617-629 (1974), O. Voegeli et al disclose a bubble lattice device wherein a great deal of magnetic bubbles are closely arranged in columns and rows. In this device, each magnetic bubble is surrounded by a domain wall having wall magnetization modified in accordance with information. In other words, information is memorized in each magnetic bubble by modifying the wall magnetization in response to the information, instead of using presence or absence of each magnetic bubble. The bubble lattice device has increased storage density as compared with the ion-implantation.
However, a certain distance should be left between two adjacent ones of the magnetic bubbles in consideration of interaction between the magnetic bubbles. Therefore, there is a limitation about improvement of the storage density.